Most of conventional plastic products, especially plastic packaging materials are discarded soon after use, and their disposal problems are pointed out. Among general purpose packaging plastics, as representative ones, polyethylene, polypropylene, polyethylene terephthalate (hereinafter abbreviated to "PET"), etc. can be cited. But these materials are high in heat buildup when burned and there is a possibility of damaging the incinerator during burning treatment. Further, polyvinyl chlorides, which are large in the amount of use even now, cannot be burned due to their self-extinguishing properties. Also, in many cases, plastic products including such materials which cannot be burned, are buried. But due to their chemical and biological stability, they scarcely decompose but remain, thus causing a problem that they shorten the life of burial sites. Therefore, plastic products that are low in heat buildup during burning, decompose in soil, and safe are desired, and many researches are being made.
As one example, there are polylactic acids. For polylactic acids, the heat buildup during burning is less than half that of polyethylene, and hydrolysis proceeds naturally in soil or water and then they are decomposed by microorganisms into unharmful materials. Now researches are being made for obtaining molded products, specifically film sheets and containers such as bottles using polylatic acids.
Polylactic acid is a polymer formed by condensation-polymerizing a lactic acid. Lactic acids have two kinds of optical isomers, i.e. L-lactic acid and D-lactic acid. Their crystallizability varies with the ratio of structural units. For example, a random copolymer of which the L-lactic acid to D-lactic acid ratio is 80:20 to 20:80 has no crystallizability. In other words, it is a transparent, completely amorphous polymer which softens near its glass transition point of 60.degree. C. On the other hand, for a homopolymer made up of only L-lactic acid or D-lactic acid, although its glass transition point is likewise 60.degree. C., it becomes a semicrystalline polymer having a melting point of 180.degree. C. or over. The semicrystalline polylactic acid turns into an amorphous material that excels in transparency, by rapidly cooling after melt extrusion.
By the way, it is known that the strength and shock resistance of polylactic acids improve by biaxially orienting them. But such a melted and rapidly cooled cast film is very brittle and inconvenient to use as it is. Also, though it can be formed into a bag by heat-sealing and melt-sealing, it allows no elongation at the sealed portion, so that it is liable to tear.
Polylactic acid has only 3-8% elongation when pulled. It is known that it is a brittle material. If it is formed into a film, it is difficult to use without stretching. Thus, it has been tried to improve shock resistance by adding several parts by weight of another aliphatic polyester (Japanese Patent Publication 9-111107). But if these films are left at a temperature slightly higher than room temperature, there is a problem that physical properties such as elongation at break or heat-sealing strength change with time.
The shock resistance can be approximately inferred from the elongation when the film is pulled. For example, elongation of high-molecular films that excel in shock resistance is 500% or over for high-density polyethylenes, low-density polyethylenes and polypropylene, and 50-400% for aromatic polyesters such as PET and nylon. For polystyrenes, crystalline polystyrene (GPPS) as a monomer has an elongation of only 5% or under. But high-impact polystyrenes (HIPS) formed by copolymerizing butadiene have an elongation of 15-50%. Even hard polyvinyl chlorides have an elongation of several tens percent by adding a plasticizer and a shock improver. Many of shock-resistant films have an elongation of at least 10% or over. Films having a greater shock resistance have an elongation of 50% or over.
On the other hand, as biodegradable films having flexibility, films comprising a condensation polymer of an aliphatic multifuntional carboxylic acid and an aliphatic multifunctional alcohol can be cited. One example is a film comprising an aliphatic polyester having diols and succinic acid or adipic acid or both of them as a main structural unit. Such aliphatic polyester films are very supple, high in both tensile elongation and shock resistance, excel in heat-sealability, and can be used in the form of bags.
But for the above-described aliphatic polyesters, it is difficult to suppress the growth of crystals even if they are cooled rapidly after melt-extruding, because both the glass transition point and the crystallization point below room temperature, so that they become opaque. When commercial products are put in bags made of such an aliphatic polyester, they will not be clearly seen and the display effect will be poor. Further, another problem is that the film is too soft.
For example, if another film, paper or metallic foil is laminated on the film, the film is drawn and stretched during the manufacturing steps, and problems such as out-of-register and non-uniform laminating can arise.
A plastic film has been sought in which brittleness has been improved, which is not too soft, which has practically satisfactory physical properties, which has stable heat-sealability with time, and which has degradability in natural environment.